Make ifconfig(8) set and display preference numbers for IPv6
addresses. Make the kernel support SIOC[SG]IFADDRPREF for IPv6
interface addresses.
In in6ifa_ifpforlinklocal(), consult preference numbers before
making an otherwise arbitrary choice of in6_ifaddr. Otherwise,
preference numbers are *not* consulted by the kernel, but that will
be rather easy for somebody with a little bit of free time to fix.
Please note that setting the preference number for a link-local
IPv6 address does not work right, yet, but that ought to be fixed
soon.
In support of the changes above,
1 Add a method to struct domain for "externalizing" a sockaddr, and
provide an implementation for IPv6. Expect more work in this area: it
may be more proper to say that the IPv6 implementation "internalizes"
a sockaddr. Add sockaddr_externalize().
2 Add a subroutine, sofamily(), that returns a struct socket's address
family or AF_UNSPEC.
3 Make a lot of IPv4-specific code generic, and move it from
sys/netinet/ to sys/net/ for re-use by IPv6 parts of the kernel and
ifconfig(8).